I once dabbled with the AI objects as well, to try to get a stealth element to work.
Couldn't get it to work either: ran into the same issue as are describing.

So I abandoned the AI completely and went with some conditions and other triggers to simulate stealth.
Like, condition that Lara is running and/or her PLS is on, then activate more enemies.
Like that.
Works in my setup, but I do not know if something like this could be used for your level.

That said, I'm also curious to know if other people know some more about the AI and how to use it for stealth, because my current setup is not exactly what I wanted. I'd like to be able to build in more stealth elements if possible.

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If it walks like a duck and if it quacks like a duck, it is a duck.